Fix the Convert to scalar to not insert dead loads in the store case. The

load is needed when we have a small store into a large alloca (at which 
point we get a load/insert/store sequence), but when you do a full-sized
store, this load ends up being dead.

This dead load is bad in really large nasty testcases where the load ends
up causing mem2reg to insert large chains of dependent phi nodes which only
ADCE can delete.  Instead of doing this, just don't insert the dead load.

This fixes rdar://6864035

llvm-svn: 91917

1 files changed, 12 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/ScalarReplAggregates.cpp b/llvm/lib/Transforms/Scalar/ScalarReplAggregates.cpp
index 884345d4a97..79bb7c547ca 100644
--- a/llvm/lib/Transforms/Scalar/ScalarReplAggregates.cpp
+++ b/llvm/lib/Transforms/Scalar/ScalarReplAggregates.cpp
@@ -1373,11 +1373,16 @@ void SROA::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, uint64_t Offset) {
     
     if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
       assert(SI->getOperand(0) != Ptr && "Consistency error!");
-      Value *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
+      Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
       Value *New = ConvertScalar_InsertValue(SI->getOperand(0), Old, Offset,
                                              Builder);
       Builder.CreateStore(New, NewAI);
       SI->eraseFromParent();
+      
+      // If the load we just inserted is now dead, then the inserted store
+      // overwrote the entire thing.
+      if (Old->use_empty())
+        Old->eraseFromParent();
       continue;
     }
     
@@ -1397,11 +1402,16 @@ void SROA::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, uint64_t Offset) {
           for (unsigned i = 1; i != NumBytes; ++i)
             APVal |= APVal << 8;
         
-        Value *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
+        Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
         Value *New = ConvertScalar_InsertValue(
                                     ConstantInt::get(User->getContext(), APVal),
                                                Old, Offset, Builder);
         Builder.CreateStore(New, NewAI);
+        
+        // If the load we just inserted is now dead, then the memset overwrote
+        // the entire thing.
+        if (Old->use_empty())
+          Old->eraseFromParent();        
       }
       MSI->eraseFromParent();
       continue;